Hypo eliminator bath



Patented Mar. 7, 1944 HYPO' ELIMINATOR BATH Lowell E. Muehler and GeorgeT. Eaton, Rochester, N. Y., assignors to Eastman Kodak Company,Rochester, Y., a corporation of New Jersey No Drawing. ApplicationOctober 30, 1941,

Serial No. 417,164

13 Claims.

'- sions coated on paper, film, or glass supports, if,

after fixation, the hypo (usually sodium or ammonium'jthiosulfate') isnot completely eliminated from theprocessed material by'washingor othermeans, under suitable conditions of temperature and humidity duringstorage, the silver image will tend to fade" orchange color.

This fading is a result of the conversion of more or less of the silverimage to silver sulfide by the sulfur present in the residual hypo, andis manifest by a change in hue of the image first to yellowish brown,then to yellow and, in most cases, the change is accompanied by ayellowing of the unexposed portions of the image. yellowing of thehighlights is a result either of (a) the use of an exhausted fixingbath, or (b) insufiicient fixation whereby complex silversodiumthios'ulfates are retained and, under the proper conditions, decomposeto give yellow silver sulfide.

In addition to attack of the silver image by hypo within the gelatinlayer, many external agents are also effective, the most significantbeing hydrogen sulfide which is present in coal gas (illuminating gas).High humidity and temperature accelerate this reaction tremendously.Sulfur dioxide and other acid gases in the presence of the hypo of theimage layer also accelerate the fading" of the image.

The rate at which a silver image fades depends upon many factors,including (1) the concentration of hypo (or tetrathionate) in the imagelayer, (2) the concentration of hydrogen sulfide and othegaid gases inthe atmosphere, (3) the grain size 6* the silver image, and (4) thetemperature and humidity of storage.

Tests have shown that the degree of fading in a given time is roughlyproportional to the concentration of hypo up to a certain limit, andacontent as low as 0.005 milligram per square inch may cause fading withfine-grained images, especially in the case of papers.

An increase in the humidity, temperature, or both, accelerates the rateof fading, and a combination of high humidity and high temperature,which conditions usually exist in tropical countries, is fatal to aphotographic print containing hypo.

The presence of saline matter and acidic gases This in the atmospherealso tends to increase the rate of fading. 7

Since fading or sulfiding of the image must necessarily take placeinitially at the surface of the image grains, fine-grained emulsionswill tend to fade much more rapidly than coarser-grained emulsions and,in practice, chloride paper emulsions give images which are much moresusceptible to fading than bromide emulsions. Similarly, a fine-grainedpositive transparency is much more susceptible Ibo fading than an imageon a high-speednegative emulsion.

So-called hypo eliminators, which are oxidizing agents for sodiumthiosulfate in photographic films, plates, and prints, such as thehypochlorites, iodine, potassium permanganate, and hydrogen peroxide,are well known in photography but all of them sufier from disadvantages.

For example, the hypochlorites and potassium permanganate attack theimages of fine-grain materials, such'as prints and fine-grain films,when they are used in sufficient concentrations and acidities to rapidlyoxidize hypo. The same is true with iodine, and, to a less extent, withhydrogen peroxide (in the absence of alkali). Both of these are open tothe objection that they oxidize hypo principally to tetrathionate whichcan act as an agent in promoting image fading as is well known withhypo.

We have found that when a material such as sodium chlorite is used forvthe elimination of hypo, the principal product of the oxidation of thehypo is sulfate rather than tetrathionate, which is readily soluble andmay be washed out easily.

Solutions of this chemical, 0.08 and 0.16 molar, with respect to sodiumchlorite have-been tried at pH values approximately 10.15 and 10.45respectively and found to have slight effectiveness in eliminating'hypo.When the product is employed with an acid or acid salt such as acetic orpotassium acid phthalate, oxidation of thiosulfate to sulfate occurs,the rate increasing as the pH decreases.

In the presence of a buffer salt, or salt and an acid, to give a pHvalue in the approximate range of 3.0 to 7.0 but preferably 5.0 to 7.0,oxidation ofthiosulfate to sulfate occurs with sulficient rapidity andthe usual moderately washed prinlt image is not appreciably attackedduring the oxidation. The solution is sufiiciently stable under theseconditions for all practical purposes. Solutions with pH 3.0 to 5.0 maybe used with more resistant photographic images and particularly if theprints are previously bathed in an alkaline solution, e. g. 1% sodiumcarbonate, 2% sodium metaborate, or a solution containing 1% sodiumcarbonate and 1% sodium bicarbonate to decrease the hypo concentration.

Solutions of sodium chlorite at pH 4.5 to 12.0 may also be used assingle treating solutions or with two-bath treatments in conjunctionwith dilute acid solutions (e. g. 1 per cent acetic acid) used eitherbefore or after the sodium ohlorite treatment. Either the sodiumchlorite or the acid solution or both-may be buffered or not asrequired.

Acidifled solutions of sodium chlorites may also be used for oxidizingthiosulfates and other oxidizable sulfur compounds in solutions. The useof buffer combinations enables the control of rate of oxidation of thesulfur compounds and prevents too rapid decomposition of thesodiumchlorite since acid is generated in the reaction with sulfur compounds.

The source of sodium chlorite preferably used in our invention, is thecommercial material Textone" containing about 77 to over 86 per cent ofsodium chlorite, and small quantities of chlorate, chloride andcarbonate, believed to have little if any effect in the process of thepresent invention. In place of sodium chlorite, chlorites havingsuitable cation, such as other alkali metal, may be used.

Exmtn 1 Textone (80 per cent sodium chlorite grams 9.0 Acetic acid ormono, di, or trisodium phosphate to give pH grams 3.0 to 12.0 Water toliter 1 Prints, negatives, etc, are treated for one to thirty minuteswith the above solution and washed five to ten minutes to eliminatehypo, in the manner of our invention.

EXAMPLE 2 Solution A Textone (80 per cent sodium chlorite) 9.0 gramsdissolved in 600 cc. of water is adjusted to an approximate pH of 5.0with acetic acid.

Solution B EXAMPLE 3 Textone grarns Borax do 28 Succinic acid do Waterto liter 1 This mixture buffered at an approximate pH of 5.0 was used totreat partially washed prints (e. g. washed to 30 minutes) treating for5 minutes, followed by a final Wash of 5 minutes. Prints partiallywashed and leached in dilute alkali and then partially washed may alsobe treated 5 minutes with the above solution and washed 5 minutes to ridthem of hypo. This leaching treatment is of value to avoidconcentrations of hypo in the vicinity of the image which can causereduction of the image with especially the grained images.

Exmn: 4

Textone grams 10.0 Borax do 32.0 Succinic acid do 13.4 Water to liter1.0

This mixture is buffered at an approximate pH of 5.5. Instructions foruse are as for Example 3. Exmu 5 Textone grams 5 to 10 Sodium acetate(anhydrous) do 32 Acetic acid (glacial) cc 24 Water to liter 1 For use:Wash prints 20 to 30 minutes, leach for 5 minutes in one of thefollowing solutions:

(a) 1% sodium bicarbonate (b) 2% sodium metaborate (NazBaOa8HaO) (c) 1%sodium carbonate and 1% sodium bicarbonate.

Then treat for 5 minutes in the hypo eliminator solution and wash 5minutes.

EXAMPLE 6 In case it is desired to package a dry mixture of chemicalsfor use in hypo elimination baths in the manner of our invention, amixture of the following composition may be used.

In separate parts of a two-compartment package:

Part A Textone grams 10 Part B Borax grams 32 Succinic acid do 14Dissolve the chemicals of Part B in 1 liter of water followed by thechemicals of Part A. For use refer to instructions for Example 4.

Treatment of prints having excessive content of hypo will causephotographic reduction of the image. For this reason it is essentialthat the prints be washed 20 to 30 minutes in running water Or partiallywashed and the hypo content further lowered by leaching in a dilutealkali bath as previously indicated.

Introduction of excessive quantities of hypo into the hypo eliminatorformation of more or less chlorine dioxide giving a bright yellow colorto the solution. To avoid the formation of chlorine dioxide in thesolution add (to any of the Examples 3, 4, and 5) precipl tated silveror finely divided silver powder or filings, for example 1 to 2 grams perliter, which acts as an acceptor to free the solution of chlorinedioxide.

It is to be understood that the disclosure herein is by way of exampleand that we consider as included in our invention all modifications andequivalents falling within the scope of the appended claims.

What we claim is:

1. A hypo eliminator bath for use in the elimisolution will resultinthe" nation of hypo from photographic silver images comprising alkalimetal chlorite.

2. A hypo eliminator bath for use in the elimination of hypo fromphotographic silver images comprising a bufifered solution of alkalimetal chlorite.

3. A hypo eliminator bath for use in the elimination of hypo fromphotographic silver images comprising a solution of alkali metalchlorite buffered to a pH within the range of 3 to 12.

4. A hypo eliminator bath for use in the elimination of hypo fromphotographic silver images comprising a solution of sodium chloritebuffered to a pH within the range of 3 to 5.0.

5. A hypo eliminator bath for use in the elimination of hypo fromphotographic silver images comprising a solution of sodium chloritebufiered to a pH within the range of 5.0 to 'I.

6. A hypo eliminator bath for use in the elimination of hypo fromphotographic silver images comprising a solution of sodium chloritebuffered to a pH within the range of 3 to 5.0 with acetic acid.

'1. A hypo eliminator bath for use in the elimination of hypo fromphotographic silver images comprising a solution of sodium chloritebuffered to a pH of 5 with a salt and an acid.

8. The method for the elimination of hypo from photographic silverimages, which comprises treating the images with alkali metal chloritesolution.

9. The dry chemicals packaged for use in making a hypo eliminator bathby dissolving in water for use in the elimination of hypo frophotographic silver images, comprising alkai metal chlorite.

10. The dry chemicals packaged for use in making a hypo eliminator *bathby dissolving in I water for use in the elimination of hypo 'iromphotographic silver images, comprising alkali metal chlorite and abufiering compound.

11. The method of eliminating hypo from photographic silver images in agelatin layer, which comprises treating the layer, after exposure,development, and fixing with hypo, with a solution of an alkali metalchlorite.

12. The method of eliminating hypo from photographic silver images in agelatin layer, which comprises treating the layer, after exposure. de-

velopment, and fixing with hypo, with a solution

